The present invention relates to an alkaline storage battery of high capacity density exhibiting superior cycle stability.
With the recent development of semiconductor technology, compact and light-weight multifunctional electric appliances have been developed progressively with rapid realization of compact personal equipment of portable type represented by note-book type personal computers. Therefore, there is an increasing demand even for the alkaline storage battery that has a wide application as the power source of such equipment to have a compact and light-weight design.
Up to date, the main active material for the positive electrode of alkaline storage battery has been nickel oxide (NiOOH). Concerning the electrode substrate per se, industrialization of an electrode made of a three-dimensional foamed nickel porous material with a higher porosity (95%) into which a nickel oxide powder is filled at a high density (foamed metal type electrode) in place of a sintered type electrode using a conventional sintered substrate (Japanese Examined Patent Publication No. Sho 62-54235; U.S. Pat. No. 4,251,603; etc.) has led to drastic improvements of the energy density of such nickel positive electrode.
To the realization of high energy density nickel positive electrode, improved manufacturing method of the nickel oxide powder as the active material is an important contributory technology. Any conventional manufacturing method of the nickel oxide powder has adopted a process where an aqueous nickel salt solution is reacted with an aqueous alkaline solution such as sodium hydroxide to precipitate nickel hydroxide, which is then aged to grow crystals and subsequently ground with some mechanical grinding method. This method has drawbacks that it is not only tedious but also does not readily produce a high packing density due to irregular shapes of the powders obtained by this method. Therefore, an alternative manufacturing method was proposed that reacts an aqueous nickel salt solution with ammonia to form an ammonium complex of nickel with which an aqueous alkaline solution is further reacted to grow nickel hydroxide (Japanese Examined Patent Publication No. Hei 4-80513). This method has enabled not only cost-effective continuous production of nickel hydroxide but also high density packing because of the resembling shape of the obtained powders to a sphere.
However, the use of high density particles of a large size obtained by this method which have grown up to a size of dozens of xcexcm as an active material produces a problem of impairment of charge/discharge efficiency due to low electronic conductivity of the active material. This problem has been coped with by an improvement to supplement electronic conductivity by adding either Co or its oxide, or Ni, etc. to the active material (Japanese Examined Patent Publication No. Sho 61-37733; Electrochemistry, Vol. 54, No. 2, p. 159 (1986); Power Sources, Vol. 12, p. 203 (1988)). Other improvements have also been attempted to incorporate an additional metallic element other than Ni, such as Cd or Co, into the active material to increase the charge/discharge efficiency (Japanese Examined Patent Publication No. Hei 3-26903; Japanese Examined Patent Publication No. Hei 3-50384; Electrochemistry, Vol. 54, No. 2, p. 164 (1986); Power Sources, Vol. 12, p. 203 (1988)). Furthermore, because of a demand for a cadmium-free battery from the aspect of environment, there are a proposal of Zn as an exemplary substitution metallic element for Cd on the one hand and a proposal of incorporation of three elements, Co, Zn and Ba, on the other hand (U.S. Pat. No. 5,366,831). Such incorporation of different metallic elements into nickel oxide and forming a solid solution in order to realize a charge/discharge characteristic of high efficiency is a long known art (Japanese Laid-Open Patent Publication No. Sho 51-122737, etc.)
Improvements in the shape, composition and additive of the electrode substrate and the active material as discussed above have drastically increased the energy density of the positive electrode and, at present, even such a positive electrode as having an energy density of about 600 mAh/cc has become commercially practical. As mentioned previously, however, there is an increasingly expanding tendency of the demand for the alkaline storage battery to have a more increased energy density for use as a power source of compact portable equipment. In order to realize more increased battery energy density, approaches from various aspects including positive electrode, negative electrode, electrolyte, separator and their structure may be contemplated. Concerning the negative electrode, by an actual use of metal hydride with high energy density (Power Sources, vol. 12, p. 393 (1988)) in place of the conventional cadmium negative electrode, a volume energy density doubling or more than that of the positive electrode has been attained. Moreover, concerning the battery structure, high energy density has been realized rapidly with the technical developments of thin separators, high density packing of active material into electrode substrate, etc., which have now almost reached their limits.
Under the circumstances, in order to realize even higher energy density, realization of a higher energy density of the positive electrode which occupies almost half the volume of a battery has been taken as playing a significant role as the most effective elementary art.
Estimated approaches for realization of improved energy density of the positive electrode may include an improvement of the active material tap density, a reduction of the amounts of any additive and a reduction of metal contents in the foamed nickel substrate. These technologies, however, are almost reaching their limits. This necessitates an attempt to modify the active material per se to improve its reactivity and reaction order. The current positive electrode active material nickel oxide is xcex2-Ni(OH)2 (bivalent oxide) upon production of a battery which has been considered to develop a reaction of one electron exchange (utilization=100%) with xcex2-NiOOH (trivalent) during normal charge/discharge operation. When exposed to overcharge, however, xcex2-NiOOH in charged state can be partially oxidized to xcex3-NiOOH (3.5- to 3.8-valent) which is a high order oxide. Such xcex3-NiOOH has been known to be at least a nonstoichiometric material of disordered crystal structure (J. Power Sources, Vol. 8, p. 229 (1982)). Conventionally, the xcex3-NiOOH is electrochemically inactive and induces not only reductions of voltage and capacity but also various hazardous events such as contact failure of the active material with a conductive material or the substrate due to expansion of electrode volume resulting from a broadening interlayer space, separation of the active material from the substrate, depletion of the electrolyte due to intake of water molecules by the active material. This has led to attempts of various measures to best suppress generation of undesirable xcex3-NiOOH.
However, in order to realize a higher energy density by using a nickel oxide-based active material, a good use of the high order oxide xcex3-NiOOH is of much importance. For such purpose, there is a proposed material having a similar structure to that of xcex1-hydroxide wherein Ni is partially replaced with a different metal such as Mn (III), Al (III) or Fe (III) and anions and water molecules are incorporated between the layers (J. Power Sources, Vol. 35, p. 294 (1991); U.S. Pat. No. 5,569,562; Japanese Laid-Open Patent Publication No. Hei 8-225328; and others). It has been considered that this oxide readily develops charge/discharge reaction with a high order oxide having a mimicking structure to that of xcex3-NiOOH. Another proposal of using charge/discharge between xcex1-phase and xcex3-phase has been disclosed in the U.S. Pat. No. 5,348,822. In fact, however, this oxide is a material having wide interlayer gaps and an extremely bulky density, making it difficult to perform high density packing. This may indicate poor practical utility of this oxide.
As an alternative, it has been attempted to provide a coating of a cobalt oxide on the active material surface to improve electronic conductivity and charge/discharge efficiency in order to realize high energy density of the positive electrode. Although different from the above-mentioned technique that uses reaction up to the xcex3-phase, this technique can produce marked improvements in the utilization and positive electrode energy density, compared with the conventional positive electrode made of a mere mixture with cobalt or cobalt oxide.
To the contrary, the present inventors have newly discovered an active material that develops charge/discharge reaction with the high order oxide xcex3-NiOOH (Abstracts of Autumn Congress of Association of Electrochemistry, p. 181 (1995)) and noted it as a novel active material. As one example, the inventors proposed modification or reformation of nickel oxide by incorporating therein an additional different metallic element for the purpose of high density and high order reaction as one example (Japanese Laid-Open Patent Publication No. Hei 10-149821). Incorporation of Mn into nickel oxide in particular enabled marked improvement of the charge/discharge efficiency with the best use of xcex3-phase. The present inventors also elucidated that control of the valence of Mn in the nickel oxide material facilitates high order reaction larger than a valence of 1.2 and proposed a synthesizing method for realizing high density (Japanese Laid-Open Patent Publication No. Hei 10-011071; Japanese Laid-Open Patent Publication No. Hei 10-053225). In addition to the present inventors, some inventors have proposed such nickel hydroxide that can cycle between xcex2-phase and xcex3-phase reversibly (WO 98/20570).
On the other hand, in order to improve various characteristics of alkaline storage battery including higher energy density, better cycle life and so on, it is very important to improve the separator. The characteristics required for the separator for use in alkaline storage battery may include better affinity for any electrolyte, excellent rate of absorption and retention capacity of electrolyte, superior alkali resistance to tolerate repeated charge and discharge, operations for a long time, and good gas permeability generating in the battery. To date, the separator for use in alkaline storage battery has been made of a nonwoven fabric sheet comprising polyamide fibers, polyolefin fibers, etc. Particularly, the polyolefin fiber, which reduces self-discharge of the battery and exhibits better alkali resistance at use in a high temperature range, has been adopted widely. However, the separator made of a polyolefin fiber sheet is inferior in affinity for any electrolyte and poor in electrolyte retention thereby manifesting a depleting tendency of the electrolyte when it is exposed to repeated charge and discharge for a long time. Therefore, treatment of such polyolefin fiber nonwoven fabric sheet to impart hydrophilicity thereto has been studied. Exemplary treatments include: (a) oxidation using fuming sulfuric acid or concentrated sulfuric acid; (b) graft treatment with a monomer having a hydrophilic group; (c) plasma treatment. Any of these treatments can improve electrolyte retention markedly and facilitates reduction of electrolyte depletion comparatively. There is also a proposal to fix an ion exchange fine powder onto the separator surface (Japanese Laid-Open Patent Publication No. Hei 9-330694).
As the aforementioned ion exchange fine powder, any ion exchange resin, metal oxide or hydroxide or their inorganic salts have been considered to prove effective. Such ion exchange fine powder which has a high ion exchanging ability can adsorb and capture metallic ions such as manganese ion, iron ion, aluminum ion, etc. as well as ammonium ion, chloride ion, nitrate ion, sulfate ion, etc. all of which have been taken as impairing factors of battery performance and can well preserve an alkaline electrolyte. Therefore, the separator fixed with the ion exchange fine powder has been considered to enable alleviation of self-discharge by the shuttle effect, reduction of electrolyte depletion thereby realizing a long life battery. According to this proposal, the use of a binder resin for fixing the ion exchange fine powder onto the separator prevents physical separation or chemical decomposition of the ion exchange fine powder as a result of which the effect can be maintained for a long time.
As discussed before, attempts have been made to improve charge/discharge efficiency and elevate the reaction order with the use of nickel hydroxide with a coating of a cobalt oxide material or a solid solution or eutectic mixture nickel hydroxide material with Mn incorporated therein as the positive electrode active material. However, the use of cobalt oxide-coated nickel hydroxide is prone to produce xcex3-phase in a competitive manner with oxygen evolution reaction at the end of charge operation due to repeated charge and discharge, compared to the positive electrode prepared by simply mixing an additive cobalt oxide with nickel hydroxide, although it improves charge/discharge efficiency and realizes high energy density. As described previously, the xcex3-phase is prone to be accumulated inside the electrode during repeated charge/discharge operations, because it is electrochemically inactive and difficult to be discharged. This results in swelling of the electrode and an increase of the specific surface area of the electrode as well as intake of the electrolyte contained in the separator by the positive electrode, which produces another problem of easy development of electrolyte depletion. This in turn produces a problem of slightly poor cycle stability of the resultant battery. To the contrary, the solid solution or eutectic mixture nickel hydroxide material with Mn incorporated therein uses the reactions (from the very initial charge/discharge cycle) up to the xcex3-phase during normal charge and discharge. As a result, the active material is swollen and constricted markedly during a charge/discharge cycle, resulting in an increasing tendency of the specific surface area of the electrode. Therefore, even when this active material is used, the electrolyte in the separator readily moves into the positive electrode which also produces a problem of easy development of electrolyte depletion. Therefore, this nickel hydroxide has a drawback of slightly poor cycle stability compared to the conventional nickel hydroxide in which generation of xcex3-phase is suppressed and about one electron exchange can proceed.
A primary object of the present invention is to provide an alkaline storage battery having a high energy density and an excellent cycle life characteristic from which the above-mentioned various problems have been cancelled by improving the separator.
The present invention provides an alkaline storage battery comprising a positive electrode including a nickel hydroxide material as an active material, a negative electrode, a separator and an electrolyte, wherein the nickel hydroxide material of the positive electrode contains at least Mn or is disposed with a coating of a cobalt oxide material on its surface and wherein the separator carries particles of a hydrophilic and insulating metal oxide directly attached to its surface with no aid of binder.
Here, the metal oxide particle is preferably at least one selected from the group consisting of titanium oxide, aluminum oxide, zirconium oxide, potassium titanate, tungsten oxide, and zinc oxide, and most preferably anatase type TiO2.
It is desirable for the metal oxide particle to have a mean size of 0.01 to 0.1 xcexcm.
It is also preferable for the metal oxide particle to be carried on the separator in an amount of 0.1 to 20 wt % with reference to the separator weight.